1. Field of the Invention
The present invention relates to an optical recording apparatus, such as a laser printer, for emitting laser beams from light sources, and deflecting and scanning the laser beams for optically recording an image.
2. Description of the Related Art
A laser printer or other conventional optical recording apparatus, such as that disclosed in Japanese Patent Application Publication No. 2004-325859, is configured to guide a laser beam to an optical fiber array having a plurality of optical fibers, and to focus and scan the laser beams emitted from the optical fiber array on a scanning surface. In this type of apparatus, the optical fiber array and the scanning surface are arranged in an optical conjugate. Since the pitch of beam spots focused on the scanning surface is greater than the pitch of adjacent scan lines, a row of beam spots is focused on the scanning surface at a slant when scanned to form scan lines.
FIG. 1 shows the relationship of this optical fiber array and the beam spots focused on the scanning surface. In FIG. 1, 2ωF is the mode field diameter of the laser beam emitted from each optical fiber in the array; PF is the pitch of optical fibers forming the optical fiber array; 2ωD is the spot diameter of the beam spots focused on the scanning surface (while the actual spot diameter varies according to the magnitude of aberration in the optical system, spot diameter denotes the diffraction-limited spot diameter in this case); PD is the pitch of scan lines on the scanning surface; M is the magnification of the optical system; and θ is the sloped angle of the row of beam spots focused on the scanning surface. The mode field diameter described above is the size (full width) of 1/e2 the central light intensity (13.5%) for the light intensity distribution of beams propagated over the optical fibers. Below, PD and 2ωD are represented in Equations (1) and (2).
Equation (1)PD=MPF·sinθ  (1)
Equation (2)2ωD=2ωF·M   (2)
If 2ωF is 3.5 μm, PF is 125 μm, 2ωD is 50 μm, and PD is 21.2 μm when using a short-wavelength semiconductor laser having no greater than a 450-nm wavelength as the light source, we obtain M=12.5 times and θ=0.680° from Equations (1) and (2).
From Equation (1), it is clear that error in the sloped angle θ of the beam spots focused on the scanning surface result in error of the scanning line pitch PD on the scanning surface. Further, the smaller the sloped angle θ, the smaller the allowable error. In the above example, an error in the sloped angle Δθ=±0.0068° corresponds to an error of ±1% in the PD. Therefore, it is necessary to achieve rigorous precision in the sloped angle θ to suppress error in the scan line pitch.
Next, a method will be described for alleviating the sensitivity of the scan line pitch error ΔPD which corresponds to the sloped angle error Δθ of the beam spots on the scanning surface.
One such method is to increase the sloped angle θ of beam spots formed on the scanning surface. From Equation (1), the pitch PF of optical fibers in the optical fiber array may be reduced to increase the sloped angle θ.
Generally, optical fibers have a two-layered structure including an outer peripheral portion called a clad formed primarily of quartz glass, and a center portion called a core formed by doping the quartz glass with germanium. Since the size of the light-propagating region (core) of the optical fiber is normally smaller than the clad portion, Japanese Patent Application Publication No. 2000-147296 discloses a method of reducing the PF in the optical fiber array by etching the outer diameter of the clad portion in the optical fibers. However, thinning the clad portion by etching the outer diameter thereof invites irregularities in the thickness of the clad portion. A method of bonding or the like thinned clad parts to a groove-shaped substrate, such as that disclosed in Japanese Patent Application Publication No. 2001-066438, for fixing optical fibers has poor workability in forming an optical fiber array. Another method well known in the art adjusts the rotation of the optical fibers as a preliminary step to fixing the fibers in order to arrange the core part along a straight line, but this method also results in poor workability.
Another method for reducing error in the scan line pitch PD caused by sloped angle error Δθ uses means to reduce the magnification M of the optical system. However, as can be seen from Equation (2), the magnification M of the optical system is a coefficient associated with the mode field diameter 2ωF of beams emitted from the optical fibers and the spot diameter 2ωD for the beam spots focused on the scanning surface and, hence, 2ωD is reduced when reducing M. To avoid this problem, a method well known in the art provides the optical system with an aperture member for focusing the beams. However, the aperture member blocks portions of the beams, resulting in loss of light quantity.
Vertical cavity surface emitting lasers (VCSEL) don't have a large output and cannot produce a short-wavelength laser at the present moment. Specifically, the wavelength of a vertical cavity surface emitting laser is limited to the near-infrared range (about 780 nm), giving the optical power per element an upper limit of a few mW (2-3 mW).
In contrast, blue (405 nm wavelength) and red (633-680 nm wavelength) edge-emitting lasers have been produced with a power per element of 30-60 mW. Accordingly, edge-emitting lasers provide more freedom in selecting wavelengths and are superior to surface-emitting lasers as a high-power light source. However, as described above, it is very difficult to remove the effects of the error Δθ in the sloped angle of beam spots focused on the scanning surface in edge-emitting lasers, which is the greatest technological problem in optical elements combining edge-emitting lasers and optical fiber arrays.